The present invention is directed to structure and method of thermoelectric device package according to certain embodiments. More particularly, some embodiments of the invention provide structure and method for reducing thermal stresses in thermoelectric legs during manufacturing a thermoelectric device. Merely by way of example, it has been applied for using CTE (coefficient of thermal expansion) matching ductile materials at the interface between thermoelectric materials and shunts on ceramic base plate. It would be recognized that the invention has a much broader range of applicability.
Additionally, the present invention is directed to thermoelectric composite structures and methods of making the same according to some embodiments. More particularly, certain embodiments of the invention provide a bulk thermoelectric composite material capped with two metal layers forming a structure for the manufacture and assembly of thermoelectric elements for large-scale thermoelectric power systems. Merely by way of example, the invention presents a method of co-sintering powdered thermoelectric composite materials with powdered metal material on top and bottom to form a bulk metal-capped thermoelectric sandwich structure capable of making a plurality of thermoelectric elements that may be sorted by thermoelectric performance for the manufacture of a custom-scaled thermoelectric module and are mechanically and electrically robust. However, it would be recognized that the invention has a much broader range of applicability.
Thermoelectric devices are often packaged using a plurality of thermoelectric legs arranged in multiple serial chain configurations on a base structure. Each of the plurality of thermoelectric legs is made by either p-type or n-type thermoelectric material. The thermoelectric (TE) material, either p-type or n-type, is selected from semiconductor characterized by high electrical conductivity and high thermal resistivity. One p-type leg is pairwisely coupled to one n-type leg via a conductor from each direction in the serial chain configuration, one conductor being coupled at one end region of the leg and another conductor being coupled at another end region of the leg. When the thermoelectric device is applied with a bias voltage across the top/bottom regions using the two conductors as two electrodes, a temperature difference is generated so that the thermoelectric device can be used as a refrigeration (e.g., Peltier) device. When the thermoelectric device is disposed to a thermal junction with conductors at first end regions of the legs being attached to a cold side of the junction and conductors at second end regions of the legs being in contact with a hot side of the junction, the thermoelectric device is able to generate electrical voltage across the junction as an energy conversion (e.g., Seebeck) device.
The energy conversion efficiency of thermoelectric devices can be measured by a so-called thermal power density or “thermoelectric figure of merit” ZT, where ZT is equal to TS2σ/k where T is the temperature, S is the Seebeck coefficient, σ is the electrical conductivity, and k is the thermal conductivity of the thermoelectric material. In order to drive up value of ZT of thermoelectric devices utilizing the Seebeck effect, on the one hand, efforts are made for searching high performance thermoelectric materials and developing low cost manufacture processes, on other hand, efforts also are needed for improving thermoelectric leg packaging techniques whenever the any high-performance thermoelectric materials are available. Additionally, there are needs for improving manufacturability of the high performance bulk-size thermoelectric legs that are packaged into thermoelectric devices.
For example, different types of materials may be provided for forming p or n type legs and for forming cold or hot-side contacts. During a package process to assemble them together, large thermal stresses during device operation would be a key problem to overcome, e.g., when more thermoelectric applications push the hot-side temperature exceeding 600° C. Further, bond strength between different thermoelectric materials in package would also be one of the big issues, as newly developed thermoelectric material combinations and new operation environment requirements raise more challenges as well as opportunities.
For example, the thermoelectric legs made of or including either n-type or p-type thermoelectric materials integrated with conductive material as an electrical contact at both ends before the legs (on a size scale of a few millimeters cubed) can be diced from a larger raw pellet. Conventionally, a series of sputtered or evaporated conductive thin films are formed on a thermoelectric material to make electrical contact to the thermoelectric elements for assembling into a module. There are multitudes of challenges with such thin films on various thermoelectric materials with high performance properties. Most issues include film delaminating and cracking or the whole piece of thermoelectric material cracking during the formation of individual thermoelectric legs or elements. The thin film electrical contacts mentioned above are used for contact and bonding but they are too thin by themselves to allow direct measurement of the electrical properties of the thermoelectric material. The thin film does not provide low enough electrical resistance to allow for sufficient current spreading across the leg cross-section. This results in a high resistance measurement and one that could differ depending on where on the surface the measuring probe makes contact.
It is commonly known that the properties of thermoelectric materials vary throughout a large sample such that small subsamples diced from the as-manufactured large sample have a wide range of properties compared to those measured from the large sample. Many thermoelectric materials are mechanically brittle, prone to cracking as thickness of the manufactured pellet changes, and hard to dice into the final thermoelectric legs with desired form factor without breaking Even though a relatively large piece of thermoelectric material with metal contacts can be made, its electrical properties can only be estimated on average as a whole piece of material without being able to properly determine the individual leg performance after dicing. For a thermoelectric material with thin film electrical contacts, binning of the thermoelectric legs by selected performance level is not possible prior to building the full thermoelectric system.
Merely as an intermediate material, the thin film electrical contacts mentioned above also suffer from poor electrical conduction due to film cracking, parasitic resistances, and bond failure, leading to poor electrical integration when assembling into thermoelectric power generation systems. Alternatively, co-sintering of metal foils with thermoelectric materials has been attempted, but the metal foils often experience problems of poor bonding with the thermoelectric material.